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I'm planning to pour a pad into an existing open carport / garage area that has mid-span supports resting on poured piers and a foundation wall going around the outside. I'm planning on digging down about 10-12", laying gravel and compacting, laying sand and compacting, then add a vapor barrier and metal mesh / rebar.

Is there anything I need to think about when pouring the new slab up to the edges of the foundation and around the piers? do I need to wrap the slab edges with foam insulation to give an expansion joint / allow them to slip past each other when settling occurs or is there a better solution?

The piers / foundation have been in place for at least 40 years. The slab would just be adding a new surface in the garage area.

EDIT: there will eventually be living space above the garage. I was sort of assuming I'd want the vapor barrier due to this. The space is approx 30' wide by 20' deep, and there's three garage slots, with posts supporting the joists above between each slot.

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    A vapor barrier is a good idea to keep the floor dry and the garage less humid. I suggest the vapor barrier be placed on a layer of sand (2"-3"), so it wouldn't be punctured by the sharp edges of the gravel. – r13 May 6 at 21:27
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I assume the pier is above the finished floor. You are correct in thinking to provide the full depth expansion joint around the pier, as well as around the perimeter of the slab that will be poured against existing concrete.

If the garage area is large, in addition to the expansion joint, it is recommended to have a control joint at an approximate 10'-15' interval in each direction to minimize the potential for random crack. The control joint (1/8"-1/4" wide x 1" depth) shall be made by saw-cut within 24 hours of concrete placement. Around the pier, the diamond-shaped control joint is recommended to avoid diagonal cracks at the reentrant corners of the slab.

The last recommendation is the fresh concrete shall have at least 5% of entrained air to improve its durability against weathering, especially in the cold regions.

enter image description here

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  • I have never saw cut a crack line. I always do it just as Isherwood mentions I have used 12’ as a guide slicing through the mix after floating but prior to hard trowel has worked well for me. But I have poured over the footings and had expansion material around the posts. If the footings are above additional may be helpful.+ – Ed Beal May 6 at 22:18
  • @Ed Beal It's a good practice too. The saw cutting is usually done on a project with significant floor areas. I agree that it might not be practical for residential slabs. – r13 May 6 at 23:05
  • I haven’t done any really large slabs or large enough with piers through the slab since my dad passed and have never needed more than crack lines but I did up vote as I know it is needed on much larger slabs than this. – Ed Beal May 7 at 12:55
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I'm not a concrete specialist, but I've seen many garages and basement slabs poured and observed the outcomes in an extreme environment with seasonal temperature swings of over 120° F. Virtually none had the need for expansion joints (compressible material) around the perimeter. Concrete shrinks as it cures, and it doesn't expand appreciably later unless subject to extreme heat. A carport offers shade, so that's unlikely. It can't really hurt, but it does leave a somewhat ugly and fuzzy gap around the edges.

That said, the interior piers are a different story. The size of a hole is reduced as the material around it shrinks. That means those openings will get tighter. I'd use a thin (1/4" or so) expansion buffer around them. Foam sill seal would be great. You can leave it high and trim it later if you like.

I'm much more concerned with control joints. Nearly all slabs crack, so you want to define where that happens for the best long-term appearance. Cutting 1/3-depth on a 10' grid is a good strategy, but you'll probably want to adjust for symmetry and uniformity.

Around the post footings I'd make extra cuts, particularly from the corners. I would try to center the intersection of your larger grid on the post, and cut a diamond similar to r13's image. I'd cut all the way (or as close as you can come) to the footing, though. Cracks will propagate all the way to those points.

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  • Fairly good point. One thing to note, some people call the "expansion joint" the "contraction joint", actually it works both ways that allow the free movement (expand/contract) of the slab to relieve the stress due to edge constrain, which is the main cause of the cracks. – r13 May 6 at 21:20
  • I agree and @10-12’ control lines for the cracks and nothing around the premier. If you plan on building above make sure to put in all the footings you need now and photo document them or the inspector may need an engineer to sign off on your plans (expensive). + – Ed Beal May 6 at 22:14

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